1. Field of the Invention
This invention relates to a pixel structure comprising thin film transistors (TFT) such as a liquid-crystal projector. In particular, it relates to improvement in light shielding in an active matrix type of liquid-crystal display for a light valve where liquid crystal is switched by a TFT. This invention also relates to a method for manufacturing the pixel structure.
2. Description of the Prior Art
A variety of displays using a liquid-crystal panel has been recently developed as a wall-hung or projection-type TV or a display for an OA device. Among liquid-crystal panels, an active matrix type of liquid-crystal display where a thin film transistor as an active element is incorporated in a liquid-crystal display is promising in providing a high-quality display for an OA device or a display for a high definition because increase in the number of scanning lines does not adversely affect contrast or a response speed. It may allow large display to be easily achieved in a projection-type of liquid-crystal display such as liquid-crystal projection.
In a common active matrix type of liquid-crystal display for a light valve used for liquid-crystal projection, an intense light is radiated to a small device for switching liquid crystal by a TFT to turn ON/OFF per a pixel; a transmitted light is controlled, depending on image data; and the transmitted light is extendedly projected on a screen via an optical system such as a lens. When an active layer in the TFT is made of polysilicon (p-Si), a leak current during OFF may be generated in a channel of the TFT by photoexcitation not only due to an incident light but also due to a reflected light from an optical system such as a lens.
In such a conventional active matrix type of liquid-crystal display for a light valve, gate lines 8 and data lines 10 are arranged in a matrix form, i.e., they are orthogonal to each other. A transparent electrode such as ITO 18 is formed in a region delimited by the gate and the data lines while a TFT is formed in a crossing of a gate line 8 with a data line 10 as illustrated in FIG. 11. FIG. 12 is an enlarged view of the area encircled in FIG. 11, showing a TFT-forming area. In the data line 10, there is formed a data-line/TFT contact 16 for supplying a signal to a source electrode 13. A drain electrode 14 is connected to ITO 18 as a pixel electrode via an ITO/TFT contact 17. An LDD region 15 is formed between the channel (the area covered by the gate line.) of the TFT and a source-drain region. FIG. 13 shows cross sections (a) and (b) taken on lines F-Fxe2x80x2 and G-Gxe2x80x2 of FIG. 12, respectively. In the figure, a film for blocking a light from a rear face (referred to as a xe2x80x9cback shield filmxe2x80x9d) 3 via a base insulating film 2 and a black matrix 12 above the TFT are formed on a transparent insulating substrate such as a glass substrate 1. Thus, when a light enters through a liquid-crystal layer from the side of an opposed substrate to the TFT, the black matrix 12 blocks the incident light (IL) while the back shield film 3 blocks a reflected light (RL) from an optical system.
The black matrix 12 may be formed on the same substrate as the TFT via an interlayer film as illustrated in FIG. 13 or on the opposed substrate to the TFT via the liquid-crystal layer. When the black matrix 12 is formed on the opposed substrate to the TFT, it must be larger than the back shield film 3 in the light of misalignment by about 10 xcexcm as a precision in superposing the two substrates. As a result, an opening ratio cannot be increased.
To date, the black matrix is, therefore, always formed on the same substrate as the TFT. In such a structure, a large margin as described above is not necessary because a higher alignment precision can be achieved employing a semiconductor device manufacturing method. However, since positional relationship between two shield films and the TFT is not taken into consideration, a light due to irregular reflection within a panel is not been sufficiently blocked. In particular, in the gate-line forming region as shown in FIG. 13(b), there are formed the back shield film 3 and also the black matrix 12, which are adequate to block a light. On the other hand, in a region around the pixel electrode as shown in FIG. 13(a), the back shield film 3 and the black matrix 12 are limited in their widths for improving a pixel opening ratio. An incident light from the edge of the black matrix 12 is, therefore, reflected on the surface of the back shield film 3 in the region around the pixel electrode in the polysilicon channel between the source and the drain electrodes 13, 14 and the LDD region 15, and a reflected light from the edge of the back shield film 3 enters into the LDD region 15. These reflected lights might cause current leak. Of course, the incident and the reflected lights contain not only directional components parallel to the gate line as described above, but also various directional components, among of which may enter the channel region under the gate line.
Of course, although the widths of the back shield film and the black matrix can be increased to prevent the incidence or reflected light from entering the channel, it leads to reduction in a pixel-opening ratio.
Thus, an object of this invention is to provide a pixel structure for a light valve where a pixel opening-ratio can be as large as possible, while preventing an incident light from a substrate surface or a reflected light from an optical system from entering the channel.
This invention provides a TFT (thin film transistor) based pixel structure comprising a back shield film formed on a transparent insulating substrate; a TFT consisting of a polysilicon channel formed on the back shield film with an interlayer film formed therebetween, a gate insulating film and a gate electrode connected to a gate line; a data line for transmitting a data signal to the TFT; and a black matrix for blocking an incident light to the TFT, wherein a dummy contact hole not reaching the back shield film is formed at least in the interlayer film on the back shield film near the lateral face of the TFT along the longitudinal direction of the channel within the region delimited by the back shield film and the black matrix, and a film made of at least an interconnection material is formed on the side wall of the dummy contact hole.
In the above pixel structure, it is preferable to form the dummy contact hole before forming the gate line and to deposit a gate-line material in the dummy contact hole simultaneously with forming the gate line, or to form the dummy contact hole before forming the data line and to deposit a data-line material in the dummy contact hole simultaneously with forming the data line.
This invention also provides a TFT-based pixel structure where the back shield film is formed on the transparent insulating substrate in a matrix form and is wider than the other interconnections only in the region to which the channel and the LDD are projected.
This invention also provides a pixel structure where the TFT is formed in the crossing of the gate line with the data line, and the dummy contact hole is formed at four corners of the crossing.
This invention also provides a method for manufacturing a TFT (thin film transistor)-based pixel structure comprising forming a back shield film, the first interlayer film, a polysilicon to be a channel of the TFT, a gate insulating film, a gate line including a gate electrode, the second interlayer film, a data line, the third interlayer film and a black matrix on a transparent insulating substrate in sequence, wherein after forming the gate insulating film and before forming the gate line, a dummy contact hole not reaching the back shield film is formed in the gate insulating film and the first interlayer film on the back shield film near the lateral face of the TFT along the longitudinal direction of the channel within the region delimited by the back shield film and the black matrix, and a film made of a gate-line material is formed on the side wall of the dummy contact hole simultaneously with forming the gate line, or after forming the second interlayer film and before forming the data line, a dummy contact hole not reaching the back shield film is formed in the second interlayer film, the gate insulating film and the first interlayer film on the back shield film near the lateral face of the TFT along the longitudinal direction of the channel within the region delimited by the back shield film and the black matrix, and a film made of a data-line material is formed on the side wall of the dummy contact hole simultaneously with forming the data line.
In particular, it is preferable to form the back shield film with a conductive material, to form the contact hole for controlling a potential of the back shield film by multiple etching steps, and to form the dummy contact hole simultaneously with at least one of the contact-hole etching steps.
This invention also provides a method for manufacturing a TFT-based pixel structure comprising a back shield film where the back shield film made of a conductive material is formed in a matrix form for controlling its potential, and is wider than the other interconnections only in the region to which the channel and the LDD are projected.
According to this invention, a dummy contact hole is formed near the lateral side of the channel and a film made of an interconnection material is formed within the dummy contact hole. Thus, an incident light from the edge of the black matrix or a reflected light through the edge of the back shield film from the rear face of the substrate are blocked by the interconnection material film and therefore do not reach the channel of the TFT, resulting in prevention of leak current due to photoexcitation. Furthermore, the shield film is wider only under the channel of the TFT to effectively block a reflected light from the rear face. Consequently, required areas of the black matrix and the back shield film can be minimized to avoid reduction in an opening ratio.
In addition, forming the dummy contact hole simultaneously with forming the other contact hole can eliminate an additional step for forming a dummy contact hole, resulting in minimizing manufacturing cost increase.